ifupdown2.conf: vlan_aware_bridge_address_support: allow ip on vlan-aware bridge...

ifupdown2.conf: vlan_aware_bridge_address_support: allow ip on vlan-aware bridge (closes #58)

In linux its possible to assign an ip address to a vlan-aware bridge
For some use cases is it useful to restrict users from configuring ips on
bridges VA. This patch will let admins and distributions decide if it is
necessary to warn the user in such case.

The patch introduces a new configuration variable in:
    /etc/network/ifudpown2/ifupdown2.conf

vlan_aware_bridge_address_support: yes|no|on|off|0|1 (default to yes)

[8:30:41] root:~ # cat /etc/network/ifupdown2/ifupdown2.conf | grep "vlan_aware_bridge_address_support"
[8:30:43] root:~ # ifquery bridge
auto bridge
iface bridge
	bridge-ports swp1
	bridge-vlan-aware yes
	address 10.10.10.10

[8:30:46] root:~ # ifup bridge --syntax-check
[8:30:52] root:~ # echo $?
0
[8:30:54] root:~ # echo "vlan_aware_bridge_address_support=no" >> /etc/network/ifupdown2/ifupdown2.conf
[8:31:11] root:~ # ifup bridge --syntax-check
warning: bridge: ignoring ip address. Assigning an IP address is not allowed on bridge vlan aware interfaces
[8:31:14] root:~ # echo $?
1
[8:31:17] root:~ #
[8:31:20] root:~ # ifup -a
[8:31:22] root:~ # echo $?
0
[8:31:25] root:~ # ifquery bridge -c
auto bridge
iface bridge                                                        [fail]
	bridge-vlan-aware yes                                       [pass]
	bridge-ports swp1                                           [pass]
	address 10.10.10.10                                         [fail]

[8:31:29] root:~ # ifdown bridge && ifup bridge -v |& grep "bridge vlan aware interfaces"
info: bridge: ignoring ip address. Assigning an IP address is not allowed on bridge vlan aware interfaces
[8:31:57] root:~ #

Reviewed-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>